(a) Technical Field
The present disclosure relates to a fuel cell bipolar plate and a method for manufacturing the same. More particularly, it relates to a fuel cell bipolar plate and a method for manufacturing the same, in which a carbon or metal fuel cell bipolar plate is preferably surface-treated with a complex transition metal oxide, which is a main component of a variable resistance heating element having a negative temperature, so that the bipolar plate can suitably ensure a sufficient amount of heat, required to prevent product water from freezing, by itself and in a short time and substantially without any external energy, thus improving the cold start performance of a fuel cell vehicle at a temperature below zero.
(b) Background Art
In a polymer electrolyte fuel cell, a fundamental component capable of generating electricity is a unit cell, which is defined as a separate electrical circuit comprising a polymer electrolyte membrane electrode assembly (MEA), a gas diffusion layer, and a bipolar plate (or separator).
As shown in FIG. 1, the unit cell preferably has a structure in which a bipolar plate 10a having a fuel flow field 50 and a bipolar plate 10b having an air flow field 60 are separated from each other by a polymer electrolyte membrane electrode assembly 30 and 40, and a gas diffusion layer 20 is preferably interposed between the bipolar plates 10 and 10b and the polymer electrolyte membrane electrode assembly 40, 30a and 30b, respectively, thus serving the function of diffusing reactant gases and serving as an electron transfer path.
The polymer electrolyte membrane electrode assembly 40, 30a and 30b located in the center of the unit cell is a component part preferably in the form of a thin film, in which carbon powder carrying platinum catalyst is suitably dispersed into a permeable membrane material, and has a structure in which a fuel electrode 30a and an air electrode 30b are bonded to both sides of a polymer electrolyte membrane 40, respectively.
The bipolar plates 10a and 10b located on the outside of the unit cell supply hydrogen and air required for an electrochemical reaction of the fuel cell to the polymer electrolyte membrane electrode assembly 40, 30a and 30b through the fuel and air flow fields 50 and 60, respectively, and at the same time serves as a conductor that connects an anode of the unit cell to a cathode thereof in series and as a structural support. Moreover, the bipolar plates 10a and 10b are generally formed of a carbon or metal material in terms of electrical conductivity and processability.
The operation principle of the fuel cell having the above-described configuration can be explained as electric current generation by the movement of free electrons due to the electrochemical reaction between hydrogen and oxygen.
Preferably, during operation of the polymer electrolyte fuel cell, an oxidation reaction, in which hydrogen is dissociated into hydrogen ions and free electrons, takes place in the fuel electrode 30a of the polymer electrolyte membrane electrode assembly, and a reduction reaction, in which the hydrogen ions transferred from the fuel electrode 30a to the air electrode 30b through the polymer electrolyte membrane 40 and the free electrons moving from the fuel electrode 30a to the air electrode 30b through an external circuit meet oxygen, takes place in the air electrode 30b. Accordingly, during the electrochemical reactions, electrical energy, gaseous or liquid water, and thermal energy are produced, which can be represented by the following formulas:Fuel electrode (anode): 2H2→4H++4e−Air electrode (cathode): 4H++4e−+O2→2H2O
Overall reaction in the fuel cell:2H2+O2→2H2O+electrical energy+heat energy
The electrical energy generated from the above overall reaction formula in the fuel cell is calculated from a difference between thermodynamic Gibbs free energy of water produced by the reaction and that of reactants such as hydrogen and oxygen in accordance with the following formula 1:
                    E        =                  -                                                    Δ                ⁢                                                                  ⁢                                                      g                    _                                    f                                                            2                ⁢                F                                      ⁢                                                  [            V            ]                                              [                  Formula          ⁢                                          ⁢          1                ]            
wherein E represents the equilibrium potential, F represents the Faraday constant, and gf represents the thermodynamic Gibbs free energy in a standard state.
Preferably, if the reaction energy is all converted into electrical energy in the fuel cell, as seen from formula 1 the theoretical standard equilibrium potential of the fuel cell is approximately 1.2 V.
Suitably different from the ideal energy conversion process, thermal energy of about 40% to 50% of the required power is incidentally produced in the electrochemical reaction process of the actual fuel cell. If a voltage loss (overpotential) other than electrical energy is all converted into thermal energy in the electrochemical reaction process of the unit cell and, if applying an energy balance formula, accordingly the amount of heat can be quantitatively calculated from the following formula 2:Qheat=(Videal−Vcell)×Icell×Arx[W]  [Formula 2]
wherein Qheat represents the amount of heat due to a voltage loss, Icell represents a current density of a unit cell, Vcell represents the potential of a working electrode, Videal represents the ideal reversible potential, and Arx represents the electrode reaction area.
In general, when considering that the working electrode potential is preferably 0.6 to 0.7 V and the operational temperature is preferably 70 to 80° in the vehicle fuel cell, it can be seen that the amount of heat due to the difference between the ideal reversible potential and the working electrode potential is about 50% of the required power.
In order to control the amount of heat, the fuel cell has a structure in which heat is suitably removed by a coolant flowing through coolant supply channels 70 of the bipolar plates 30a and 30b provided per unit cell as shown in FIG. 1.
In particular, the vehicle fuel cell requires a power of several tens of kW and a voltage of several hundreds of V to drive a vehicle motor and, at the same time, the amount of heat due to the voltage loss of the fuel cell corresponds to 50% of the required output.
Accordingly, the vehicle fuel cell is preferably fabricated in the form of a stack in which several hundreds of unit cells including the bipolar plates having the coolant supply channels are preferably successively stacked.
Preferably, the polymer electrolyte membrane inserted into the center of each of the unit cells contains a sulfonic acid group, which is a functional group suitably capable of exchanging cations and, thus, if the polymer electrolyte membrane contains water, hydrogen ions can migrate through the polymer electrolyte membrane by hydration of the sulfonic acid group.
Accordingly, if the polymer electrolyte membrane loses water and becomes dry, the hydration of the sulfonic acid group does not occur. As a result, the hydrogen ion transfer resistance is suitably reduced, and thereby the performance of the fuel cell stack is deteriorated.
For example, when the fuel cell vehicle is initially started at a temperature below zero, the water content of the polymer electrolyte membrane is as low as 3 to 5% compared to the water content at normal temperature. Accordingly, the performance of the vehicle fuel cell is considerably reduced by a sudden increase in hydrogen ion transfer resistance.
Further, in the case where the fuel cell stack is operated at a temperature below zero, the water produced by the electrochemical reaction in the fuel cell freezes in the polymer electrolyte membrane electrode assembly, the gas diffusion layer, etc., and thereby the transfer of the reactant gases is hindered, which results in a reduction in power density of the fuel cell stack.
Accordingly, in the case where the fuel cell vehicle is initially started at a temperature below freezing point, the start-up time is suitably delayed until the rated power is reached due to the low power density of the fuel cell stack.
In order to develop techniques to put the fuel cell vehicles to practical use, it is preferable to develop a technique or method for improving cold start performance of the fuel cell vehicle, preferably in which a level of close to about 50% of the rated power can be reached within about 30 seconds, preferably with a minimum energy of less than about 5 MJ during the initial start-up of the fuel cell vehicle at a temperature of −20° C.
Preferably, in order to stably operate the fuel cell system at a temperature below freezing point, it is necessary to develop a technique or method for coping with the freezing problem, which can suitably improve the cold start performance, prevent water in the fuel cell stack from freezing, and reduce the cold start time during the development of the fuel cell vehicle.
Accordingly, a technical method, in which an instantaneous heat load is applied to the fuel cell system so that the fuel cell stack reaches its normal operational temperature range within a very short time, may be suitably employed. Preferably, the thermal energy required to prevent product water in the fuel cell from freezing can be calculated by the following formula 3:Qrequired=Qlatent+Qsensible={dot over (m)}pwHfusion+{dot over (m)}pwcpΔT[W]  [Formula 3]
wherein Qrequired represents the thermal energy required to improve cold start performance, Qlatent represents the latent heat accompanying a phase change, Qsensible represents the sensible heat accompanying a temperature change without a phase change, and ΔT represents the difference between the external temperature and the internal temperature.
Hfusion represents the heat of fusion required to convert ice into liquid water, i.e., thermal energy of 333.6 J/g, and {dot over (m)}pw represents the mass flow rate of water produced in the fuel cell by the electrochemical reaction and can be calculated from the following formula 4:
                                          m            .                    p          w                =                              (                                          I                cell                                            2                ⁢                F                                      )                    ·                      MW                          H              ⁢                                                          ⁢              2              ⁢              O                                                          [                  Formula          ⁢                                          ⁢          4                ]            
wherein MWH2O represents the molecular weight of water.
Preferably, according to the conventional techniques, an external heating element is additionally provided in the fuel cell to supply thermal energy required during the initial start-up of the fuel cell vehicle at a temperature below freezing point, or high temperature coolant is circulated until the fuel cell stack reaches the appropriate temperature range.
The above conventional techniques supply external energy by the use of external equipment, and so the size and complexity of the fuel cell system are increased to deteriorate the efficiency of the overall system, and the time required for the fuel cell stack to reach its normal operational temperature range is increased, which increases the cold start time.
Accordingly, the cold start problem of the fuel cell vehicle is directly linked to the improvement of the durability of the fuel cell stack, and it is necessary to provide an alternative approach to overcome the current technical limitations in order to put the fuel cell vehicles to practical use.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.